Polymer electrolyte composition for improving overcharge safety and lithium battery using the same

ABSTRACT

A polymer electrolyte composition for improving overcharge safety and a lithium battery using the same are provided. The polymer electrolyte composition includes acrylate, epoxy or isocyanate at both of its terminals, and includes a compound containing an aromatic group such as thiophene, biphenyl or furan in an amount of 0.1% to 20% by weight based on the amount of the overall organic electrolytic solution. The polymer electrolyte composition further includes at least one of polyethylene glycol diacrylate (PEGDA), polyethylene glycol dimethacrylate (PEGDMA), and a mixture thereof. A lithium polymer battery using the polymer electrolyte composition can be suppressed from danger of ignition or explosion when the battery is overcharged due to some uncontrolled conditions, such as failure of a charger. Moreover, an additional cutoff device is not necessary, while still exhibiting good life cycle characteristics of the battery.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/277,679 filed on 23 Oct. 2002 now U.S. Pat No. 6,849,362.This related application is relied on and incorporated herein byreferences in its entirety.

CLAIM OF PRIORITY

This application makes reference to, incorporates the same herein, andclaims all benefits accruing under 35 U.S.C. §119 from my applicationPOLYMER ELECTROLYTE COMPOSITION FOR IMPROVING OVERCHARGE SAFETY ANDLITHIUM BATTERY USING THE SAME filed with the Korean Industrial PropertyOffice on 24 Nov. 2001 and there duly assigned Serial No. 2001-73571.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a lithium battery and, moreparticularly, to a polymer electrolyte composition for improvingovercharge safety, and a lithium battery using the same.

2. Related Art

According to the development of advanced electronic devices, there is anincreasing demand for small, lightweight portable electronic devices andequipment. Thus, there is a need for batteries having high energydensity characteristics so as to supply power to such devices andequipment.

A lithium battery comprises a cathode, an anode, an electrolyticsolution providing a movement path for lithium ions, and a separatordisposed between the cathode and the anode. Lithium batteries produceelectrical energy by intercalation/deintercalation of lithium ionsduring oxidation and reduction occurring at the cathode and the anode.However, in the case where a battery is overcharged due to malfunctionof a charger (for example, malfunction causing a sharp rise in voltage),excess lithium is precipitated at the cathode and excess lithium isintercalated into the anode. If both the cathode and the anode arethermally unstable, an organic solvent of the electrolytic solution isdecomposed to cause rapid heat generation, such as thermal runaway,adversely affecting safety of the battery.

To overcome the foregoing disadvantage, various attempts to suppressovercharge of a battery by changing the composition of an electrolyticsolution or adding additives to the electrolytic solution have beenproposed in the field of lithium ion batteries. For example, U.S. Pat.No. 5,580,684 to Yokoyama et al., entitled NON-AQUEOUS ELECTROLYTICSOLUTIONS AND NON-AQUEOUS ELECTROLYTE CELLS COMPRISING THE SAME, issuedon 3 Dec. 1996, discloses a method of improving safety of a battery byincreasing a self-extinguishing property of an electrolytic solution byadding phosphoric acid esters, such as trimethyl phosphate,tri(trifluoroethyl)phosphate or tri(2-chloroethyl)phosphate, to theelectrolytic solution. U.S. Pat. No. 5,776,627 to Mao et al., entitledAROMATIC MONOMER GASSING AGENTS FOR PROTECTING NON-AQUEOUS LITHIUMBATTERIES AGAINST OVERCHARGE, issued on 7 Jul. 1998, discloses a lithiumbattery having an internal electrical disconnection device, andcomprising a gas generating additive, such as thiophene, biphenyl orfuran. When the battery is overcharged, the gas generating additive ispolymerized to prevent migration of lithium ions, and the generated gasincreases an internal pressure of the battery so that the disconnectiondevice is activated, thereby causing internal shorting prior toexplosion.

Similarly, methods for enhancing battery safety are disclosed in U.S.Pat. No. 5,763,119 to Adachi, entitled NON-AQUEOUS ELECTROLYTE SECONDARYCELL HAVING SHUTTLE AGENT, issued on 9 Jun. 1998, U.S. Pat. No.5,709,968 to Shimizu, entitled NON-AQUEOUS ELECTROLYTE SECONDARYBATTERY, issued on 20 Jan. 1998, and U.S. Pat. No. 5,858,573 to Abrahamet al., entitled CHEMICAL OVERCHARGE PROTECTION OF LITHIUM ANDLITHIUM-ION SECONDARY BATTERIES, issued on 12 Jan. 1999, in which anelectrolyte includes an additive such as 1,2-dimethoxy-4-bromo-benzene,2-chloro-p-xyline and 4-chloro-anisol, and 2,7-diacetyl thianthrene,respectively.

However, such conventional additives may be polymerized even undernormal operating conditions. Also, since additional disconnections arenecessary, resultant batteries become bulky. In the case of a lithiumpolymer battery requiring a large amount of additives, life cyclecharacteristics may deteriorate.

SUMMARY OF THE INVENTION

To solve the above-described problems, it is a first object of thepresent invention to provide a polymer electrolyte composition which cansuppress danger of ignition or explosion of a battery when the batteryis overcharged due to some uncontrolled conditions, such as failure of acharger without need for an additional cutoff device, while notdeteriorating the life cycle characteristics of the battery.

It is a second object of the present invention to provide a lithiumsecondary battery using the polymer electrolyte composition.

To accomplish the first object, the present invention provides a polymerelectrolyte composition comprising an organic solvent, a lithium saltand an aromatic compound of formula (1):Y₁—X₁—Ar—X₂—Y₂  <Formula (1)>wherein

-   Y₁, Y₂=

where R₁ is hydrogen or alkyl,

-   X₁, X₂=

where n is an integer in the range of 0 to 20,

-   Ar=

where R₂ and R₃ are independently hydrogen or alkyl.

In accordance with another aspect of the present invention, there isprovided a lithium secondary battery using the polymer electrolytecomposition.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings, in which likereference numerals indicate the same or similar components, and wherein:

FIG. 1 is a graph showing the results of an overcharge test for abattery using a polymer electrolyte composition according to ComparativeExample 1; and

FIG. 2 is a graph showing the results of an overcharge test for abattery using a polymer electrolyte composition according to Example 1of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail.

A polymer electrolyte composition according to the present inventioncomprises an organic solvent, a lithium salt and an aromatic compound offormula (1):Y₁—X₁—Ar—X₂—Y₂  <Formula (1)>wherein

-   Y₁, Y₂=

where R₁ is hydrogen or alkyl,

-   X₁, X₂=

where n is an integer in the range of 0 to 20,

-   Ar=

where R₂ and R₃ are independently hydrogen or alkyl.

When the battery is overcharged so as to result in an increase ininternal voltage up to 4.2 V, the compound of formula (1) is polymerizedby electrical polymerization to increase the internal resistance of thebattery, thereby protecting the battery by cutting off the conduction oflithium ions, and improving overcharge safety. Since the compound offormula (1) includes acrylate, epoxy or isocyanate group at both of itsterminals, it can be gelled by heat polymerization and UV lightpolymerization.

According to the present invention, the compound of formula (1) ispreferably used in an amount of 0.1% to 20% by weight based on theamount of the overall organic electrolytic solution. If the content ofthe compound is less than 0.1%, the shortage gives rise to a poorfunction of preventing overcharge. If the content of the compound isgreater than 20%, the excess adversely affects battery performance.Usable examples of the compound of formula (1) include bisphenol Aethoxylate(₁E₀/phenol) diacrylate of formula (2) having acrylate groupsat its both terminals, and an aromatic group of bisphenol disposed atits center:

Also, the compound of formula (1) may be a compound of formula (3):

If a crosslinking agent such as amine, is added to the compound offormula (3), the isocyanate groups at both of the terminals arecrosslinked, thereby forming a urethane gel. The aromatic compound offormula (1) may be a compound of formula (4):

If a crosslinking agent, such as amine, is added to the compound offormula (4), the epoxy groups at both terminals are crosslinked, therebyforming an epoxy gel.

As the organic solvent for use in the polymer electrolyte composition,any organic solvent typically used for the manufacture of lithiumbatteries can be used. Examples of organic solvents useful in thepresent invention include, but are not limited to, at least one selectedfrom the group consisting of ethylene carbonate, propylene carbonate,dimethylcarbonate, diethylcarbonate, ethyhnethylcarbonate,tetrahydrofuran, sulforan and 2-methylhydrofuran. The solvent is presentin an effective amount typically used in lithium polymer batteries.

As the lithium salt for use in the polymer electrolyte composition, anylithium salt that is dissociated in an organic solvent so as to becapable of producing lithium ions can be used. Examples of lithium saltsuseful in the present invention include, but are not limited to, atleast one ionic lithium salt selected from the group consisting ofLiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiSCN andLiC(CF₃SO₂)₃, and the lithium salt is preferably present in aconcentration of 0.4 to 1.5 M.

The polymer electrolyte composition according to the present inventionmay further include at least one selected from the group consisting ofpolyethylene glycol diacrylate (PEGDA), polyethylene glycoldimethacrylate (PEGDMA) and a mixture thereof. Such polymers can form acopolymer with the compounds of formulas 1 thru 4, and they improve themechanical properties of a polymeric gel by crosslinkage and increasethe amount of moisture which the polymer electrolyte is able toincorporate therein.

The PEGDA, PEGDMA or a mixture thereof is, preferably, added in anamount of 10.1% to 10% by weight based on the amount of the overallpolymer electrolyte composition. If the content is less than 0.1%, suchdesired effects as described above are not achieved. If the content isgreater than 10%, the excess may adversely affect ionic conductivity.

In an embodiment of the present invention, the PEGDMA is an oligomerhaving a weight-average molecular weight of 200 to 10000, and the numberof ethylene oxides contained in the oligomer is, preferably, 3 to 20.

In another embodiment of the present invention, the PEGDMA is anoligomer having a weight-average molecular weight of 200 to 10000, andthe number of ethylene oxides contained in the oligomer is, preferably,3 to 20.

The polymer electrolyte composition according to the present inventionmay further include a polymeric filler, and usable examples of thefiller for improving mechanical strength of polymer electrolyte includesilica, kaolin, alumina and the like.

Also, the polymer electrolyte composition according to the presentinvention may further include a plasticizer. Examples of the plasticizeruseful in the present invention include ethylene glycol derivatives,oligomers thereof and organic carbonates. Specific examples of theethylene glycol derivatives are ethylene glycol diacetate, ethyleneglycol dibutylether, ethylene glycol dibutyrate, ethylene glycoldipropionate, propylene glycol methylether acetate and mixtures thereof.Specific examples of the organic carbonates are ethylene carbonate,propylene carbonate, diethyle carbonate, dimethyl carbonate and mixturesthereof.

A lithium battery according to the present invention includes a cathode,an anode and a polymer electrolyte composition comprising an aromaticcompound of formula (1).

The lithium battery according to the present invention may furtherinclude a porous separator, and any separator typically used in lithiumbatteries can be used without limitations. For example, a porouspolyethylene or polypropylene film less reactive with an organicsolvent, and suitable in view of safety, can be used as the separator.

A method of preparing a lithium battery according to the presentinvention will now be described in detail with reference to particularembodiments, but the invention is not limited thereto.

First, the polymer electrolyte composition according to the presentinvention is applied to an electrode or separator to form an electrodeassembly. Then, the electrode assembly is put into a battery case,followed by polymerization, thereby preparing a lithium battery. Thecomposition is gelled by heat polymerization or UV light polymerization.

The lithium battery according to the present invention is not limited inview of type, and the present invention can be applied to both primaryand secondary batteries.

To further illustrate the present invention in greater detail, thefollowing Examples will be given. However, it is to be understood thatthe present invention is not restricted thereto.

EXAMPLE 1

1-(1) Preparation of Cathode

A mixture (slurry or paste), prepared by dissolving LiCoO₂ employed as acathode active material, Super-P (manufactured by M.M.M. Corp.) employedas a conductive agent, and polyvinylidenefluoride (PVDF) employed as abinder in N-methyl-2-pyrrolidone (NMP) employed as an organic solvent,was uniformly applied onto both surfaces of an aluminum currentcollector to prepare a cathode coated with an active material, followedby drying to remove the organic solvent and compression molding using aroll press machine, thereby manufacturing a cathode of 4.9 cm in widthand 147 μm in thickness.

1-(2) Manufacture of Anode

A mixture (slurry or paste), prepared by dissolving mesocarbon fiber(MCF) (manufactured by Petoca Ltd.) employed as an anode active materialand PVDF employed as a binder in NMP employed as an organic solvent, wasapplied onto both surfaces of a copper current collector to prepare ananode coated with an active material, followed by drying to remove theorganic solvent and compression molding using a roll press machine,thereby manufacturing an anode of 5.1 cm in width and 178 μm inthickness.

1-(3) Preparation of Polymer Electrolyte Composition

LiPF₆ as a lithium salt was dissolved in a concentration of 1.15M in amixed solvent of ethylene carbonate (EC)/dimethyl carbonate(DMC)/diethyl carbonate (DEC) (volume ratio=3/3/4) to obtain a mixedsolution. To 100 g of the obtained mixed solution, there were added 4 gof bisphenol A ethoxylate(₁E₀/phenol) diacrylate of formula (2)(manufactured by Aldrich), 3 g of 3-chloroanisole as a polymerizationcatalyst during overcharge, and 0.1 g of benzoylperoxide as a catalyst,thereby obtaining a desired polymer electrolyte composition:

EXAMPLE 2

The polymer electrolyte composition was prepared in the same manner asin Example 1 except that 2 g of bisphenol A ethoxylate(₁E₀/phenol)diacrylate was used.

EXAMPLE 3

The polymer electrolyte composition was prepared in the same manner asin Example 1 except that 6 g of bisphenol A ethoxylate(₁E₀/phenol)diacrylate was used.

EXAMPLE 4

The polymer electrolyte composition was prepared in the same manner asin Example 1 except that 2 g of bisphenol A ethoxylate(₁E₀/phenol)diacrylate and 2 g of PEGDMA, having a weight-average molecular weightof 550 (manufactured by Aldrich), were used.

EXAMPLE 5

The polymer electrolyte composition was prepared in the same manner asin Example 1 except that 2 g of bisphenol A ethoxylate(₁E₀/phenol)diacrylate and 2 g of PEGDMA, having a weight-average molecular weightof 575, were used.

EXAMPLE 6

Manufacture of Lithium Polymer Battery

An 18 μm thick, porous polyethylene separator was interposed between thecathode and the anode manufactured in Example 1 and wound to prepare abattery assembly. Then, the polymer electrolyte compositions obtained inExamples 1 thru 5 were injected into a plurality of such batteryassemblies in each amount of 2.9 g to manufacture rectangular batteries,each having a capacity of 900 mAh, followed by gelling through heatpolymerization. The heat polymerization was performed at 85° C. for 3hours.

COMPARATIVE EXAMPLE 1

LiPF₆ was dissolved in a concentration of 1.15M in a mixed solvent ofEC/DMC/DEC (volume ratio=3/3/4) to obtain a mixed solution. To 100 g ofthe obtained mixed solution were added 4 g of PEGDMA to prepare apolymer electrolyte composition.

COMPARATIVE EXAMPLE 2

The polymer electrolyte composition was prepared in the same manner asin Comparative Example 1 except that 4 g of PEGDA was used instead ofPEGDMA.

TEST EXAMPLE 1

Overcharge Test

Lithium polymer batteries using the polymer electrolyte compositionobtained in Example 1 and in Comparative Example 1, respectively, werecharged at room temperature with a charging current of 500 mA (1 C) upto 4.1 V, and were further charged with a constant voltage of 4.1 V for3 hours, realizing a fully charged state of the battery. Chargingcurrent of 500 mA (1 C) was applied between the cathode and anodeterminals of the fully charged battery to be overcharged. Then, a timeperiod from an initial stage of the overcharge to a point when a currentcutoff member operates, and a highest battery temperature at thosetimings, were measured, and the results thereof are shown in FIGS. 1 and2. FIG. 1 shows that the battery is ignited due to thermal runaway.However, in the lithium polymer battery according to the presentinvention, as shown in FIG. 2, even though a high voltage of greaterthan 4.2 V is applied thereto, the voltage is never increased up to 6 Vor more, thereby preventing ignition due to thermal runaway.

TEST EXAMPLE 2

Discharge Capacity Test

The initial discharge capacities and capacity changes after 300 cyclesof charging/discharging of the manufactured lithium polymer batteriesprepared in Example 1–5 and in Comparative Examples 1–2 were measured,and the results are expressed as values relative to the initialcapacities. A charger/discharger (Maccor Co.) having a capacity of 1 Awas used. The charging and discharging cycling test was performed at 25°C. at a rate of 1 C, and the charging voltage was 2.75 to 4.2 V. Theresults are shown in Table 1.

TABLE 1 Average Average Average (1C) standard high-rate (2C) dischargedischarge capacity discharge capacity capacity (mAh) (mAh) (mAh) Example1 523 420 450 Example 2 513 410 440 Example 3 526 400 425 Example 4 505440 459 Example 5 501 440 413 Comparative 530 444 467 Example 1Comparative 527 445 471 Example 2

From Table 1, it is clear that the lithium polymer batteries using thepolymer electrolyte compositions according to the present invention aregreatly improved in suppressing overcharge and exhibit stablecharge/discharge characteristics.

When an internal voltage of a battery increases over 4.2 V due toovercharge resulting from various causes, the polymer electrolytecomposition according to the present invention forms electrical polymersin the electrolyte, thereby increasing internal resistance of thebattery and preventing conduction of lithium ions. Thus, the battery canbe protected from danger of ignition or explosion, and overcharge safetycan be improved. Also, since the battery of the present invention doesnot require a separate current cutoff device incorporated in aconventional battery, various advantages, including reduction in batterysize and manufacturing cost, can be achieved.

The aromatic compound included in the composition of the presentinvention has a high ionic conductivity in itself. Thus, even if a largeamount of the aromatic compound is used, there is no deterioration incharge/discharge characteristics and life span characteristics of thebattery, and the aromatic compound contained in the polymer electrolyteof the present invention can be advantageously applied to lithiumpolymer batteries and the like.

Although preferred embodiments of the present invention have beendescribed, it will be understood by those skilled in the art that thepresent invention should not be limited to the described preferredembodiments. Rather, various changes and modifications can be madewithin the spirit and scope of the present invention, as defined by thefollowing claims.

1. A polymer electrolyte composition, comprising: an organic solvent; alithium salt; and an aromatic compound of Formula (3):


2. The polymer electrolyte composition according to claim 1, wherein thearomatic compound is included in an amount in a range of 0.1% to 20% byweight based on the amount of the overall polymer electrolytecomposition.
 3. The polymer electrolyte composition according to claim1, wherein the organic solvent is selected from a group consisting ofethylene carbonate, propylene carbonate, dimethylcarbonate,diethylcarbonate, ethylmethylcarbonate, tetrahydrofuran, sulforan,2-methylhydrofuran, and mixtures thereof.
 4. The polymer electrolytecomposition according to claim 1, wherein the lithium salt is at leastone compound selected from a group consisting of LiPF₆, LiClO₄, LiAsF₆,LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiSCN, and LiC(CF₃SO₂)₃.
 5. The polymerelectrolyte composition according to claim 1, further comprising atleast one additive selected from a group consisting of polyethyleneglycol diacrylate (PEGDA), polyethylene glycol dimethacrylate (PEGDMA),and a mixture thereof.
 6. The polymer electrolyte composition accordingto claim 5, wherein said at least one additive is included in an amountin a range of 0.1% to 10% by weight based on the amount of the overallpolymer electrolyte composition.
 7. The polymer electrolyte compositionaccording to claim 5, wherein the PEGDA is an oligomer having aweight-average molecular weight in a range of 200 to 10000, and thenumber of ethylene oxides contained in the oligomer is in a range of 3to
 20. 8. The polymer electrolyte composition according to claim 5,wherein the PEGDMA is an oligomer having a weight-average molecularweight in a range of 200 to 10000, and the number of ethylene oxidescontained in the oligomer is in a range of 3 to
 20. 9. A lithiumbattery, comprising a cathode, an anode and the polymer electrolytecomposition of claim
 1. 10. The lithium battery according to claim 9,further comprising a porous separator.
 11. The lithium battery accordingto claim 9, wherein a content of the aromatic compound is substantiallyin a range of 0.1% to 20% by weight based on the amount of the overallpolymer electrolyte composition.
 12. The lithium battery according toclaim 9, further comprising at least one additive selected from a groupconsisting of polyethylene glycol diacrylate (PEGDA), polyethyleneglycol dimethacrylate (PEGDMA), and a mixture thereof.
 13. The lithiumbattery according to claim 12, wherein a content of said at least oneadditive is substantially in a range of 0.1% to 10% by weight based onthe amount of the overall polymer electrolyte composition.
 14. Thelithium battery according to claim 12, wherein the PEGDA is an oligomerhaving a weight-average molecular weight in a range of 200 to 10000, andthe number of ethylene oxides contained in the oligomer is in a range of3 to
 20. 15. The lithium battery according to claim 12, wherein thePEGDMA is an oligomer having a weight-average molecular weight in arange of 200 to 10000, and the number of ethylene oxides contained inthe oligomer is in a range of 3 to
 20. 16. A polymer electrolytecomposition, comprising: an organic solvent; a lithium salt; and anaromatic compound of Formula (4).


17. The polymer electrolyte composition according to claim 16, whereinthe aromatic compound is included in an amount in a range of 0.1% to 20%by weight based on the amount of the overall polymer electrolytecomposition.
 18. The polymer electrolyte composition according to claim16, wherein the organic solvent is selected from a group consisting ofethylene carbonate, propylene carbonate, dimethylcarbonate,diethylcarbonate, ethylmethylcarbonate, tetrahydrofuran, sulforan,2-methyihydrofuran, and mixtures thereof.
 19. The polymer electrolytecomposition according to claim 16, wherein the lithium salt is at leastone compound selected from a group consisting of LiPF₆, LiClO₄, LiAsF₆,LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiSCN, and LiC(CF₃SO₂)_(3.)
 20. Thepolymer electrolyte composition according to claim 16, furthercomprising at least one additive selected from a group consisting ofpolyethylene glycol diacrylate (PEGDA), polyethylene glycoldimethacrylate (PEGDMA), and a mixture thereof.
 21. The polymerelectrolyte composition according to claim 20, wherein said at least oneadditive is included in an amount in a range of 0.1% to 10% by weightbased on the amount of the overall polymer electrolyte composition. 22.The polymer electrolyte composition according to claim 20, wherein thePEGDA is an oligomer having a weight-average molecular weight in a rangeof 200 to 10000, and the number of ethylene oxides contained in theoligomer is in a range of 3 to
 20. 23. The polymer electrolytecomposition according to claim 20 wherein the PEGDMA is an oligomerhaving a weight-average molecular weight in a range of 200 to 10000, andthe number of ethylene oxides contained in the oligomer is in a range of3 to
 20. 24. A lithium battery comprising a cathode, an anode and thepolymer electrolyte composition of claim
 16. 25. The lithium batteryaccording to claim 24, further comprising a porous separator.
 26. Thelithium battery according to claim 24, wherein a content of the aromaticcompound is substantially in a range of 0.1% to 20% by weight based onthe amount of the overall polymer electrolyte composition.
 27. Thelithium battery according to claim 24, further comprising at least oneadditive selected from a group consisting of polyethylene glycoldiacrylate (PEGDA), polyethylene glycol dimethacrylate (PEGDMA), and amixture thereof.
 28. The lithium battery according to claim 27, whereina content of said at least one additive is substantially in a range of0.1% to 10% by weight based on the amount of the overall polymerelectrolyte composition.
 29. The lithium battery according to claim 27,wherein the PEGDA is an oligomer having a weight-average molecularweight in a range of 200 to 10000, and the number of ethylene oxidescontained in the oligomer is in a range of 3 to
 20. 30. The lithiumbattery according to claim 27, wherein the PEGDMA is an oligomer havinga weight-average molecular weight in a range of 200 to 10000, and thenumber of ethylene oxides contained in the oligomer is in a range of 3to 20.